1. Field of the Invention
The invention relates to a braking force control apparatus for a vehicle provided with a brake device that brakes the rotation of drive wheels of the vehicle equipped with a continuously variable transmission and adjusts the magnitude of a braking force acting upon the drive wheels on the basis of a brake depression amount.
2. Description of the Related Art
Continuously variable transmissions, which can perform shifting in a stepless manner, have recently been put to practical use with the object of reducing fuel consumption. The continuously variable transmissions are mainly of a belt type and a traction type, and a toroidal type is a typical traction type.
In a toroidal-type continuously variable transmission, two disks (at an input side and at an output side) are disposed parallel to each other and a plurality of power rollers are clamped between the disks by a strong force. When an inclination angle of power rollers changes, the ratio of rotation speeds of the two disks changes correspondingly, and the output revolution speed related to the input revolution speed is changed. In other words, the speed ratio is changed.
By contrast, in most belt-type continuously variable transmissions, shifting is performed with a metallic belt. A belt-type continuously variable transmission is typically constituted by an input pulley, an output pulley, and one metallic annular belt that is wound about the pulleys. A variable-width groove is provided on the outer circumference of the pulleys. The belt is wound within this groove located on the outer circumference.
The width of the groove in the pulleys is controlled hydraulically. As the width of the groove in the input pulley is increased, the belt is positioned closer to the center of the input pulley. In this case, the width of the groove of the output pulley decreases and the belt is positioned closer to the outer side of the output pulley. In other words, in this state, the belt has a small winding radius at the input pulley and a large winding radius at the output pulley. As a result, the revolution speed inputted from an input shaft to the input pulley is outputted upon reduction.
Where the width of the groove of the input pulley decreases, the width of the groove of the output pulley increases accordingly. In other words, the winding radius of the belt at the input pulley increases and the winding radius of the belt at the output pulley decreases. Thus, the inputted revolution speed is increased and the reduced torque is outputted.
In a state in which the belt is inserted into the grooves of the pulleys, the belt clamping force serving as a pressing force presses the belt against the pulleys. As a result, the belt and the pulleys move together due to the friction therebetween.
In such a belt-type continuously variable transmission, the belt can slip with respect to each pulley. Although the slip can be inhibited by increasing the belt clamping force, the increase in the belt clamping force also increases the power necessary to sustain the clamping pressure, increases power consumption, and causes wear of the device. Accordingly, in the conventional continuously variable transmission, the belt clamping force is adjusted on the basis of the operation state of the vehicle, thereby improving fuel consumption, while inhibiting the belt slip (see, for example, Japanese Patent Application Publication No. 2004-138199 (JP-A-2004-138199)).
However, in a certain vehicle travel mode, an excessively large inertia torque can be inputted from the drive wheels to the continuously variable transmission when a braking force based on a brake operation is generated. Such an event can produce, for example, the following results.
Let us assume that a vehicle travels on a road with a low friction such as observed on frozen roads and a driver performs an abrupt braking operation because the drive wheels have slipped. Because the reduction of speed in the drive wheels becomes very large due to the braking operation, an excessively large inertia torque is generated between the upstream side of the drive wheels and the downstream side of the continuously variable transmission. Furthermore, where a brake pedal is stepped upon when the drive wheels are separated from the ground as the vehicle travels and the drive wheels temporarily run idle, an excessively large inertia torque is generated following the abrupt speed reduction in the drive wheels.
A continuously variable transmission is constituted by members that are lower in strength than components of other devices installed on a vehicle, that is, mainly constituted by a metallic belt and pulleys. Therefore, when the aforementioned excessively large inertia torque is generated in a vehicle having a continuously variable transmission installed thereon and this torque is transmitted to the continuously variable transmission, the metallic belt can be damaged or fractured. Further, the above-described problems are explained in reference to the belt-type continuously variable transmission in which an excessively large inertia torque is transmitted to the continuously variable transmission from the drive wheels. However, in a traction-type continuously variable transmission, a problem associated with the deformation of curved surface of a disk can be considered. Therefore, the above-described problems can be said to be common to the continuously variable transmissions of both types.